The invention relates to a process for producing ceramic multicolored prints, in particular four-color and seven color prints, having improved reproduction quality. The process comprises the stages of colorimetric scanning of an image pattern by means of a color scanner, creation of a print profile and chromatic components by an electronic image processor, production of the print carrier, direct or indirect imprinting of a substrate that is capable of being fired, and firing. The invention is further directed towards an apparatus for implementing the process, the apparatus being special ceramic color sets.
Graphical reproductions are mostly carried out using four-color printing. In Europe the chromatic components and the corresponding printing inks of the four-color set are based on the Euroscale (EN 16538/539). The standardized colors of the four-color set are the chromatic colors yellow (Y), magenta-red (M) and cyan-blue (C) as well as the achromatic color black (S). The chromatic ground colors orange-red (R), violet-blue (B) and green (G) are represented by combining the primary colors Y, M and C. Since the colors underlying the Euroscale are already close to the theoretical color locus, in graphical four-color printing a high quality of reproduction is achieved. Only in rare cases, in particular for the production of graphical reproductions with more luminous colors in the color regions R, B and G, is use made of seven-color printing and the Kxc3xcppers scale which underlies it (H. Kxc3xcppers in Der Fadenzxc3xa4hler, a company publication produced by Gebr. Schmidt Druckfarben, Frankfurt, 1991).
In connection with the production of ceramic prints, substantially more unfavorable conditions are obtained in comparison with graphical prints. Namely, the available color palette in respect of ceramic colors is narrower than that for producing graphical reproductions, and the color locus and the intensity of the three ceramic chromatic colors Y, M and C do not correspond to the European standard. Since, as a rule, use has also been made of the Euroscale hitherto in ceramic multicolor printing, a reproduction true to the original was not possible. In addition, in ceramic four-color printing, not only did smears have to be put up with in the case of the ground colors Y, M and C, but even more smear resulted when reproducing luminous colors in the regions R, B and G. Further problems arise in ceramic printing by virtue of the fact that the colors underlying the multicolor set to be used can not only be selected according to their color locus and intensity but, on account of the ceramic firing immediately following the printing, stringent requirements also have to be met regarding the firing stability of the colors, the compatibility of the colors with one another, the compatibility of the colors with the flux, the dishwasher resistance and an emission of lead and cadmium that is as low as possible.
In order to produce ceramic four-color prints having better correspondence of original pattern and reproduction, elaborate corrections were necessary by employing so-called ornamental colors (E. Wagg and H. Hilgenfeld, Screen Printing, November 1995). Color-critical decorative surfaces were left open in the color separation and closed by ornamental colors, in particular those from the R, B and G regions. Such corrections had to be carried out by experienced lithographers.
It is accordingly an object of the invention to improve the quality of reproduction in the case of ceramic multicolor printing. According to a further object of the invention the color locus of the ceramic colors of the multicolor set to be used should be optimized, and the color space for ceramic multicolored prints should preferably be extended.
The first object is achieved by virtue of a process according to the present invention. The second object is achieved by virtue of the apparatus used for implementing the process, namely multicolor sets for the production of overglaze, inglaze and underglaze decorations on ceramic and porcelain as well as decorations on glass.
A process has been discovered for producing a ceramic multicolored print having improved reproduction quality. The process comprises colorimetric scanning of an image pattern by means of a color scanner that is capable of being profiled, creation of a print profile and issuing of a chromatic component for each color by an electronic image processor, production of a transparency for each chromatic component, production of the print carrier to be employed in the printing process using the transparencies, imprinting of a substrate that is capable of being fired or of an intermediate carrier using the print carriers and ceramic printing inks, and firing in a manner known as such of the directly or indirectly imprinted carrier that is capable of being fired. The process uses an electronic image processor, the algorithm of which, for the printable color range and separation, having been based on color data pertaining to color fields generated from ceramic colors of a multicolor set.
The multicolored print is preferably a four-color or seven-color print. Prints generated in ceramic seven-color printing are distinguished by a hitherto unknown, extended color space and therefore a substantially improved reproduction quality.
Colorimetric scanning of the image pattern and creation of a print profile can be effected by employing a commercially available color scanner that is capable of being profiled, including an electronic image processor that is suitable for such purposes. Software for profiling the scanners is commercially available. Over the course of profiling, the color values of an image point are registered by the scanner and converted into the actual color values by means of a matrix stored in a file of the electronic image processor, which describes the necessary error correction.
A significant feature of the process according to the invention is that color data pertaining to color fields obtained by using colors of a ceramic multicolor set, in particular of a ceramic four-color or seven-color set, were taken as the basis for creation of the computer algorithm for establishing the color space and also the separation of the image points into the special printing inks. For the purpose of creating the stated algorithm, color fields are created from ceramic colors of a multicolor set by combining ground colors that are compatible with one another from the series comprising yellow (Y), magenta (M), cyan-blue (C), red-orange (R), blue-violet (B), green (G) as well as black (S) in various mixing ratios and are gauged with respect to their particular color locus and their intensity. The color data obtained define the color space that can actually be obtained in the course of ceramic printing. The color space was taken as the basis for creating the computer algorithm for the separation. The color data of the several thousand color fields gauged form the support-points of the separation matrix. A programmer who is familiar with four-dimensional matrices and non-linear dependencies is able to ascertain the algorithm for the intermediate values from the support-points.
In current four-color printing the color combination comprises the colors M, Y and C, and in the case of seven-color printing, which results in an extended color space, the color combination comprises the colors M, Y, C, R, B and G and also, in each instance, S. By employing the color data pertaining to an image pattern that have been measured by the scanner and then profiled, and by using a computer algorithm for the separation which is ascertained on the basis of ceramic colors, the print profile that comprises the colors required for each image point and their area ratio is created by the electronic image processor.
Provided that, when printing ceramic colors, use is made of a substantially similar, preferably identical, multicolor set such as was taken as the basis for determining the computer algorithm, printing results are achieved that are substantially truer to the original. Although the problems of ceramic multicolor printing have long been known and elimination of the problems has necessitated elaborate corrections, hitherto use has surprisingly never been made of a separation matrix based on ceramic colors. Such a measure was not obvious because, when establishing a color set, not only the optical aspects but, in addition, aspects regarding the compatibility of the colors with one another, the compatibility of the colors with the fluxing agent, the Pb and Cd emissions, the dishwasher resistance and the potential for misprints had to be taken into account. Accordingly the discovery of suitable color combinations of a color set required the investigation of thousands of combinations.
The issuing or establishment, for each color, of the chromatic components that are improved in accordance with the invention by the electronic image processor, and also, from this, the production of transparencies, are effected in a known manner. By employing the transparencies it is possible, in a known manner, for print carriers to be used in the printing process to be produced, for example templates for screen printing, printing plates for offset printing or engraving plates for dabber printing. The printing itself can be effected directly onto the carrier that is capable of being fired, for instance ceramic, porcelain or glass. In the case of indirect printing, use is mostly made of the known transfer technique, whereby printing of the ceramic colors is effected first by employing the print carriers on transfer paper provided with a layer of glue or wax by way of an intermediate carrier and the decoration is then transferred on the intermediate carrier, generally after a film has been applied over the same, onto the carrier that is capable of being fired. Printing is directly followed by firing, the firing conditions conforming in a known manner to the carrier material and also to the type of decoration, such as overglaze, in-glaze or underglaze, which already has to be taken into account when selecting the multicolor set employed.